Africa's abundant large herbivores are very heterogeneously distributed, both geographically and regionally. Within a region, some localities contain dense animal concentrations although areas nearby may be virtually unoccupied. Mixed-species herds are a conspicuous feature of areas where animals concentrate. The prevailing explanations of local distributional concentrations are (1) that different herbivore species facilitate each other's foraging, and (2) that animals are protected from predation by both intraspecific and interspecific association. If facilitation of grazing were an overriding factor, mixed species herds should move extensively with localized rain showers to obtain the greatest forage yield. If predation were the major factor influencing animal densities and distributions, rapid, unpredictable spatial movements would further reduce predation. But because resident, non-migratory species tend to occupy home ranges that are stable over time, neither of these hypotheses is totally compelling. Because tropical forages are of lower quality than temperate ones and are often chronically deficient in mineral elements, I tested the hypothesis that areas where animals concentrate are localities supporting forages of higher mineral content. I report here that the mineral content of foods is an important determinant of the spatial distributions of animals within the Serengeti National Park, Tanzania. Based on ecological critieria, magnesium, sodium and phosphorus appear particularly important.